A digital wireless data transmission over a broadband mobile radio channel can be made adaptive and flexible by using the OFDM (Orthogonal Frequency Division Multiplexing) modulation method, and combined cost-effectively with multiple antenna methods (MIMO methods). A general explanation of multicarrier methods and especially the OFDM method is found, for example, in nt.eit.uni-kl.de/forschung/ofdm.html.
In general, the spatial signal processing for the application of MIMO methods can be carried out individually per subcarrier of a multicarrier modulation method. The MIMO OFDM transmission can thus be interpreted as parallel implementation of a number of MIMO single-carrier modulation methods. In principle, a different MIMO method can thus be utilized for each individual subcarrier.
In the existing MIMO methods, the following four categories are distinguished, in principle:
a) non-channel-oriented
b) transmitter-oriented
c) receiver-oriented
d) channel-oriented
Category a) includes the diversity methods which manage without any channel information and which only utilize the spatial diversity of the transmission without adapting themselves to the channel, and can also transmit only one data stream simultaneously in the space domain.
In case b) channel information is determined at the receiving end and, taking into consideration the signal processing at the transmitting end, an equalization and possibly a spatial signal separation is performed. This procedure corresponds to the equalization and separation of the traditional MIMO approach in which spatial multiplexing is carried out at the transmitter end without utilizing the channel information.
Category c) largely corresponds to variant b), the channel information being utilized for preequalization at the transmitting end. At the receiver, no channel information is now taken into consideration (e.g. Joint Transmission).
In case d) the signal processing at the transmitting end and at the receiving end is in each case adapted to the instantaneous complete channel information or to a statistical quantity describing the channel (e.g. spatial covariance matrix). Channel-oriented methods have the advantage that the number of data streams to be transmitted in parallel in the space domain can be selectively adapted in dependence on the channel quality already at the transmitting end. This also includes the case where only one data stream is transmitted (e.g. beam forming).
Channel-oriented MIMO methods have the potential of greatest capacity increase because they provide for an adaptation of the signal processing to the channel both at the transmitting end and at the receiving end and, at the same time, diagonalize the channel spatially in such a manner that a number of different spatial transmission modes are produced. To do this, however, the channel information must be present at the transmitter in order to be able to select or adjust the optimal transmission strategy. In the case of TDD-based methods, the channel information needed for transmitting can be determined during the receiving phase since reciprocity of the channel can be assumed. In the case of FDD-based methods, this is not immediately possible. In this case, the channel information must be determined at the receiver and signaled back to the transmitter.
Since the complete channel information comprises the entire information about the broadband channels between all transmitting and receiving antennas, signaling back requires a considerable amount of resources with not insignificant signaling effort. In the case of a MIMO OFDM transmission with N subcarriers, T transmitting antennas and R receiving antennas, N*T*R complex-valued channel coefficients must be signaled back from the receiver to the transmitter if the channel information is provided in the frequency domain.
In general, and with the correct dimensioning of the OFDM signal, the channel impulse response can be represented in the time domain with W<=N channel coefficients, where W is the number of channel coefficients required for describing the channel in the time domain, i.e. channel taps. Very frequently, W<<N applies. In this case, the channel information is transferred into the time domain and only W*T*R<N*T*R channel coefficients need to be signaled back. However, this additionally presents the problem of determining the number of coefficients W actually needed for the instantaneous channel description.
A channel coefficient can be described by a complex number. The W*T*R channel coefficients are generally transmitted individually once per frame duration for each user. Broadband systems can distinguish between slight delay differences of various propagation paths if the reciprocal bandwidth is smaller than the delay difference of the paths. It can thus be generally assumed that with the channel bandwidth, the number W of channel coefficients describing the broadband channel also rises. Independently of the choice of a single- or multicarrier modulation method (such as OFDM), at least W*T*R values must thus be transmitted per user and per frame in a system of bandwidth B, the channel of which can be described by W coefficients over this entire bandwidth.
To keep down the complexity and the signaling expenditure, only one MIMO method is generally used for all subcarriers of the OFDM signal. In methods which utilize the channel information at the transmitter, however, the full instantaneous or time-averaged or statistical channel information is always utilized for determining the transmitting strategies which are individual for each of the N subcarriers.
One representative of the methods which utilize the full instantaneous channel knowledge at the transmitter is the SVD MIMO (Singular Value Decomposition MIMO) method. Time-averaged channel information in the form of the spatial channel covariance matrix is used in Eigenbeam forming, when a number of spatial data streams are transmitted in parallel, or in MRC (Maximum Ratio Combining) beam forming when only one spatial data stream is transmitted.
In methods which transmit a number of spatial data streams in parallel, the transmission strategy for each OFDM subcarrier is described by a matrix, the dimension of which corresponds to T*R. This assumes that the OFDM system is dimensioned in such a manner that each subcarrier is subjected to so called “flat fading”. This means that the bandwidth is much less than the reciprocal temporal channel dispersion so that for each subcarrier channel, in the case of an antenna, the description by only one single complex channel coefficient is possible. In methods which transmit only one data stream, the transmission strategy per subcarrier is described completely by a vector of dimension T.
This results in the following possibilities:
1. The full channel information is signaled back in the frequency domain, e.g. N*T*R values are transmitted in the case of SVD MIMO or N*T values are transmitted in the case of channel-oriented beam forming. In the first case, up to T parallel data streams can be transmitted via different spatial modes. In the second case, only one data stream can be transmitted simultaneously in the space domain. This procedure is also possible for single-carrier modulation methods with frequency domain equalization.2. The complete channel information is signaled back in the time domain. In this context, methods are additionally applied which allow the current number of channel coefficients W required for describing a channel to be estimated. Such methods transform the channel transfer function from the frequency domain into the time domain and order in accordance with their amplitude values, for example, the N values thus obtained in the time domain.
If, in this context, a multiplicity of approximately equal smallest amplitude values occur, it is assumed that these values do not contribute to the description of the channel but describe noise. These noise values can be omitted by specifying a corresponding threshold. To determine this threshold, an SNR value to be expected can also be additionally used if it is known. The remaining number W<N of values corresponds to the estimation of the values currently required for the channel description. Before the further use of the channel coefficients, they must be brought back into their original order.
This estimation method bears the risk that the number W is wrongly determined. In addition, the receiver of the signaling information must be informed in some form which value W is currently used as a basis, e.g. after the transmission of the W*R*T values by an additional information item that the signaling of the MIMO coefficients is ended.
To bypass this problem, variant 1 is used in most cases in practice, assuming a maximum possible W=N. In general, this procedure is also possible for signal-carrier modulation methods with frequency domain equalization or time-domain equalization. In the latter case, the transformation of the transfer function from the frequency domain into the time domain is left out.
3. From time to time, spatial covariance matrices or the spatial value, averaged over time, of the channel are only signaled back for Eigenbeam forming or MRC beam forming. This variant is advantageous inasmuch as it only requires a very low mean signaling effort but it does not provide for instantaneous channel adaptation. If, in addition, MIMO methods are changed, this must be additionally reported to the respective transmitter.